1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to a method for efficiently allocating a transmission period in a transmission frame period of a wireless network system.
2. Description of the Related Art
Along with the recent development of wireless communication technology and the resulting spread of wireless equipment, there is an increasing demand for high-speed, highly reliable data transmission via a wireless link. A Wireless Local Area Network (WLAN) developed to meet the demand includes stations (STAs), which are mobile data communication equipment, and an access point (AP) capable of exchanging data with the STAs. The AP and the STAs, located in the same wireless service coverage area, are known as a Base Service Set (BSS).
Particularly, an improved WLAN system achieves high throughput by employing a Multiple Input Multiple Output (MIMO) technology that uses a plurality of transmission antennas and a plurality of reception antennas, and an Orthogonal Frequency Division Multiplexing (OFDM) technology. In this WLAN system, STAs located in one wireless service coverage area transmit or receive data using wireless resources allocated from an AP. The AP allocates the wireless resources in the form of phase resources, and the term “phase resources” as used herein refers to a period in which the STAs or AP can transmit data.
FIG. 1 illustrates a structure of a transmission frame period in a typical WLAN system.
Referring to FIG. 1, in a transmission frame period 100 having a fixed length determined in the system, an AP transmits a MAP (mapping) frame 110 indicating phase resource allocation over the full transmission frame period 100. The MAP frame 110 is composed of a downlink MAP 120 indicating a period in which the AP can transmit data, and an uplink MAP 122 indicating a period in which the STAs can transmit data. The downlink MAP 120 is composed of a Number-of-STAs field 130, and at least one STA Information field 132 determined based on the Number-of-STAs field 130. Similarly, the uplink MAP 122 is composed of a Number-of-STAs field 134, and at least one STA Information field 136 determined based on the Number-of-STAs field 134. The STA Information fields 132 and 136 each include STA ID fields 140 and 144 that indicate STAs allocated downlink and uplink periods, and Time Offset fields 142 and 146 that indicate time offsets allocated to the STAs.
An STA, allocated a period in the MAP frame 110, receives data in a period indicated by corresponding STA information, in a downlink period 112, and transmits data in a period indicated by corresponding STA information, in an uplink period 114. The STA maintains a sleep mode in the other periods except for the period in which the MAP frame 110 is transmitted and the period indicated by the MAP frame 110. A contention period 116 following the uplink period 114 can be accessed by an STA on a contention basis.
In this way, the AP estimates the amount of resources required for each STA in one transmission frame period 100, and allocates downlink and uplink periods according to the estimation. However, when the AP overestimates the amount of resources required for the STA, the wireless resources are wasted, decreasing data throughput performance. In this case, the STA may not use the allocated wireless resources. Moreover that resource cannot be reused by other STAs since that resource is already allocated to the particular STA. When the AP underestimates the amount of resources required for the STA, the STA cannot be allocated its required resources until at least the next transmission frame period, suffering transmission delay and jitter of the uplink service. The transmission delay and jitter affects a Quality of Service (QoS) required by the STA. In addition, if the STA accesses the contention period 116 due to a lack of its allocated resources, the STA cannot operate in the sleep mode for the contention period, wasting its power.
Therefore, in the wireless communication system in which the AP determines the downlink and uplink periods required for the STA through scheduling, there is a need for technology to prevent the reduction in data throughput and the waste of STA power, and accurately allocate the periods.